How to join lines not starting with specific pattern to the previous line in UNIX?

Please take a look at the sample file and the desired output below to understand what I am looking for.

It can be done with loops in a shell script but I am struggling to get an

awk

/

sed

one liner.

SampleFile.txt

These are leaves.
These are branches.
These are greenery which gives
oxygen, provides control over temperature
and maintains cleans the air.
These are tigers
These are bears
and deer and squirrels and other animals.
These are something you want to kill
Which will see you killed in the end.
These are things you must to think to save your tomorrow.

Desired output

These are leaves.
These are branches.
These are greenery which gives oxygen, provides control over temperature and maintains cleans the air.
These are tigers
These are bears and deer and squirrels and other animals.
These are something you want to kill Which will see you killed in the end.
These are things you must to think to save your tomorrow.